linux-kernel - Re: [PATCH v3 02/34] KVM: x86: hyper-v: Introduce TLB flush ring

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Message-ID: <87y1z1v9d5.fsf@redhat.com>
Date:   Mon, 16 May 2022 16:29:42 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 02/34] KVM: x86: hyper-v: Introduce TLB flush ring

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
>> To allow flushing individual GVAs instead of always flushing the whole
>> VPID a per-vCPU structure to pass the requests is needed. Introduce a
>> simple ring write-locked structure to hold two types of entries:
>> individual GVA (GFN + up to 4095 following GFNs in the lower 12 bits)
>> and 'flush all'.
>> 
>> The queuing rule is: if there's not enough space on the ring to put
>> the request and leave at least 1 entry for 'flush all' - put 'flush
>> all' entry.
>> 
>> The size of the ring is arbitrary set to '16'.
>> 
>> Note, kvm_hv_flush_tlb() only queues 'flush all' entries for now so
>> there's very small functional change but the infrastructure is
>> prepared to handle individual GVA flush requests.
>
> As I see from this patch, also the code doesn't process the requests
> from the ring buffer yet, but rather just ignores it completely,
> and resets the whole ring buffer (kvm_hv_vcpu_empty_flush_tlb)
> Maybe you should mention it here.
>
>
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/include/asm/kvm_host.h | 16 +++++++
>>  arch/x86/kvm/hyperv.c           | 83 +++++++++++++++++++++++++++++++++
>>  arch/x86/kvm/hyperv.h           | 13 ++++++
>>  arch/x86/kvm/x86.c              |  5 +-
>>  arch/x86/kvm/x86.h              |  1 +
>>  5 files changed, 116 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 1de3ad9308d8..b4dd2ff61658 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -578,6 +578,20 @@ struct kvm_vcpu_hv_synic {
>>  	bool dont_zero_synic_pages;
>>  };
>>  
>> +#define KVM_HV_TLB_FLUSH_RING_SIZE (16)
>> +
>> +struct kvm_vcpu_hv_tlb_flush_entry {
>> +	u64 addr;
>> +	u64 flush_all:1;
>> +	u64 pad:63;
>> +};
>
> Have you considered using kfifo.h library instead?
>

As a matter of fact I have not and this is a good suggestion,
actually. Let me try to use it instead of my home-brewed ring. I'll
address your other comments after that. Thanks!

-- 
Vitaly